user id should be available in commits api response
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### Description
the user id is not present in commits API, author or committer emails are present but there is no way identity authenticated(Gitlab user id) user of commit
### Proposal
```json
{
"id": "0696058f035a5631c9602bec239a61dcdf3ccccc",
"short_id": "0696058f",
"title": "commit message",
"created_at": "2017-03-22T17:38:22.000+05:30",
"parent_ids": [
"a99af2d2c3c52baf2532f74bcf9e63295545a74d"
],
"message": "commit message",
"author_name": "example",
"author_email": "example@example.com",
"authored_date": "2017-03-22T17:38:22.000+05:30",
"committer_name": "example",
"committer_email": "example@example.com",
"committed_date": "2017-03-22T17:38:22.000+05:30",
"user_id": "12"
}
```
instead of (without user id)
```json
{
"id": "0696058f035a5631c9602bec239a61dcdf3ccccc",
"short_id": "0696058f",
"title": "commit message",
"created_at": "2017-03-22T17:38:22.000+05:30",
"parent_ids": [
"a99af2d2c3c52baf2532f74bcf9e63295545a74d"
],
"message": "commit message",
"author_name": "example",
"author_email": "example@example.com",
"authored_date": "2017-03-22T17:38:22.000+05:30",
"committer_name": "example",
"committer_email": "example@example.com",
"committed_date": "2017-03-22T17:38:22.000+05:30"
}
```
### Links / references
https://docs.gitlab.com/ce/api/commits.html
issue